The present invention relates to an image forming system provided with preventive means against ozone generation and capable of forming images stably without causing fatigue deterioration in the course of repeated image formation.
In the electrophotography based on the Carlson method, images are formed by the steps of electrifying the surface of a photoreceptor, forming latent images thereon through imagewise exposure, forming toner images through the development of said latent images, and transferring and fixing said toner images onto an image receiving material. After the transferring, the photoreceptor is subjected to processes of toner removal and charge neutralization to restore it to the original state, so that it is used repeatedly for a long period of time.
Accordingly, the photoreceptor for electrophotography needs a high durability which allows repetitive image formation over a long time, besides being high enough in electrophotographic properties such as electrification property, sensitivity, and residual potential property.
Meanwhile, there is given much attention in recent years to the organic photoreceptor using an organic photoconductor which is harmless, low in cost, high in processability and has a large degree of freedom in selecting a material suitable to use conditions of a photoreceptor. And the research and development is recently focussed on a function-separating type organic photoreceptor, in which the charge-generating function is allotted to a charge-generating material (hereinafter referred to as a CGM) and the charge-transferring function is allotted to a charge-transferring material (hereinafter referred to as a CTM).
Since a variety of hole-transferring p-type CTMs of high performance have been found, predominant function-separating type organic photoconductors in recent years are those having a CGM-containing charge-generating layer (hereinafter referred to as a CGL) which constitutes the lower layer and a CTM-containing charge-transferring layer (hereinafter referred to as a CTL) which constitutes the upper layer.
In the research and development of such organic photoreceptors, however, ozone generated by an electrifying or transferring corona discharger causes undesired problems, because it damages a CTM, particularly a CTM having high electrophotographic characteristics, to deteriorate the photoreceptive layer and, moreover, it is harmful to the human body. This necessitates the use of adequate preventive means against ozone generation.
As CTMs for the above organic photoreceptor, there are known styryl compounds, hydrazone compounds, pyrazoline compounds and triazole compounds as disclosed, for example, in Japanese Pat. O.P.I. Pub. No. 48869/1986. However, these are not necessarily adequate as a CTM for making a photoreceptor equivalent to a selenium photoreceptor. Under such conditions, Japanese Pat. O.P.I. Pub. No. 225660/1988, for example, discloses cycloheptanylidene derivatives and cycloheptenylidene derivatives of specific structure as CTMs capable of providing high sensitivities.
However, the CTMs comprising those cycloheptanylidene derivatives or cycloheptenylidene derivatives of specific structure are very low in resistance to ozone generated by corona discharge; therefore, a photoreceptor having a surface CTL containing such a CTM is affected by the ozone in the course of repetitive image formation, bringing about a considerable deterioration in electrophotographic properties.
For example, the following compound, one of the typical CTMs according to the present invention, showed a considerable deterioration when exposed to ozone, in an ozone resistance test conducted by the present inventors. ##STR2##
In the above ozone resistance test, the CTM was exposed to an atmosphere comprising 100 ppm ozone for 60 minutes by repeating a 10-minute exposure six times, and then its structural deterioration was determined from the change in peak value of absorption spectrum measured with a light resistance testing fluorescent lamp having a wavelength distribution shown in FIG. 6. The results are shown in FIG. 7, in which the intensities of absorption spectra after respective 10-minute exposures are given in percentages to the peak value of unexposed CTM which is set at 100. It is understood from FIG. 7 that the relative intensity of absorption spectrum lowers to about 1/2 or less when the CTM is exposed for 30 minutes.
As is apparent from the above description, even if a CTM high in sensitivity and other electrophotographic properties is newly developed, a photoreceptor comprising such a CTM is affected by ozone and halved in performance in the course of repetitive image formation. This lessens the degree of freedom to select an appropriate CTM and thereby makes it difficult to prepare a photoreceptor having desired electrophotographic properties.
On the other hand, various approaches are used to minimize the ozone generation from the viewpoint of the image-forming system. And much attention is given to the contact method, which uses a bias-voltage-applied conductive roller for electrifying or transferring unlike the corona discharge method using a discharging wire. The contact method is effective in reducing the amount of ozone generated in the image-forming system. However, in the increasing demand for a photoreceptor with much longer durability, it is difficult to reach a satisfactory solution only by such reaction-kinetics-based counter-measures against generation and influences of ozone in the image-forming system.
As the results of studies, the present inventors have found that high image qualities cannot be stably obtained in repetitive image formation unless the ozone concentration in the system is controlled within a certain range and, in conjunction with such control, the property of a photoreceptive layer is improved, when a photoreceptor used in the image forming system has a surface layer formed of a CTM which is susceptible to ozone deterioration, though high in electrophotographic properties, especially in sensitivity.